What are the roles of bacteria in the nitrogen cycle?

The nitrogen cycle is the cycling of nitrogen between the atmosphere and organic material (the biosphere). Bacteria are responsible for interconversions in both directions. A simple outline follows. Am happy to give greater detail (names of bacterial species etc) if necessary. The main entry point for atmospheric nitrogen into the biosphere is by NITROGEN-FIXING bacteria. Some of these bacteria are found living in so-called 'root nodules' of leguminous plants, such as clover. These ultimately convert nitrogen found in air spaces in the soil into a form that the plants can use to create useful compounds, such as amino acids- the building blocks of proteins necessary for growth. (Side note: this is a symbiotic relationship - the bacteria and plant both benefit by providing each other with materials for growth). When these plants die, the nitrogen compounds can be broken down (decomposed) by other bacteria in the soil to produce ammonium ions. These ions can in turn be oxidised by free-living NITRIFYING bacteria into nitrates. Nitrates can be taken up by other plants and converted into useful compounds for growth (e.g. Amino acids). Nitrogen can be returned to the atmosphere by DENITRIFYING bacteria in the soil, which reduce nitrates back into nitrogen again.